This invention relates to a device for the calibration of an ultrasonic transducer by measurement of the reflection of plane ultrasonic waves from a rigid spherical target.
It has been assumed thus far that apparatus for the ultrasonic scanning of objects emit powers which are sufficiently low to ensure that the patients are not endangered in medical echography. In order to achieve better control in this respect (contemporary apparatus often comprise a network of transducers instead of a single transducer) and to enable in general more accurate adjustment of apparatus for the formation of images of an object by ultrasonic scanning, it is increasingly important to provide accurate calibration methods for the transducer (transducers) incorporated in this apparatus.
The paper "Calibration of imaging systems by means of spherical targets" by M. Auphan, R. H. Coursant and C. Mequio at the tenth "Symposium International d'Imagerie Medicale", held in Cannes, France, from Oct. 12 to 16, 1980, clearly states the drawbacks of the customary methods. This paper, moreover, proposes a new calibration method for transducers where the ultrasonic pressure is determined on the basis of calculations which are based on the echos received after reflection from a small, rigid sphere which is struck by the waves emitted by the transducer.
A calibration method utilizing a spherical target has already been described in the article "Reflection of a plane impulsive acoustic pressure wave by a rigid sphere" by M. Auphan and J. Matthys, published in the magazine "Journal of Sound and Vibration", 1979, 66-2, pages 227 to 237.
However, it has been found that even the finest suspension wire for the spherical target produces echos which have an amplitude which cannot be ignored and which disturb the measurements and calculations of the useful echos. The proposed use of magnetic levitation means has been found to be very problematic in practice. It has also been proposed to intercept the echos which originate from a spherical target during its free fall in the liquid medium for the propagation of the ultrasonic waves. The apparatus for this method comprises a system for retaining the sphere which is released at a desired instant by means of a small electric motor and which reflects echos to the transducer during its free fall, the transducer emitting brief pulses having a repetition frequency of approximately 1 kHz. During the fall, the time interval between the instant at which the sphere is struck by the wave and the reception of the corresponding echo becomes smaller. A comparison device which detects the duration of this time interval activates an oscilloscope which comprises a memory at the instant at which the traject of the sphere intersects the axis of the transducer, the signals thus recorded being subsequently subjected to the calculations elaborated in said publications. However, it is a drawback of this method that a significant uncertainty remains as regards the actual position of the sphere. This is because the opening of the device whereby the sphere is retained can induce turbulences in the liquid medium for the propagation of the ultrasonic waves. Such turbulences are large enough to cause a deflection of the sphere so that the traject of the sphere is no longer the traject it would follow solely under the influence of gravity.
It is an object of the invention to provide a calibration device which simply solves the problem of suspending the sphere so that the solution involving a freely falling sphere can be abandoned.